1. Field of the Invention
This invention relates to the manufacture of solid and microcellular polyurethane elastomers, the manufacturing improvement comprising the use of essentially liquid polyesters having a "true melting point" of no higher than about 30.degree.C.; said polyesters comprising products containing from about 25 percent by weight to about 70 percent by weight of the epsilon-oxycaproyl unit, and said polyurethanes being characterized by high strength and exceptional resistance toward humidity at elevated temperatures.
This invention further relates to novel liquid polyesters and to processes for their manufacture, which polyesters comprise liquid, substantially hydroxyl-terminated polyesters containing from about 25 percent by weight to about 70 percent by weight of the epsilon-oxycaproyl unit, the balance of the polyester being derived from the reaction of at least one dicarboxylic acid with at least two straight chain glycols selected from the group consisting of ethylene glycol, 1,3-propanediol, 1,4-butanediol and 1,5-pentanediol, said polyesters having a true melting point of no higher than about 30.degree.C.
The liquid polyesters of the present invention are eminently suitable as plasticizers for vinyl resins and as intermediates for the novel polyurethanes possessing improved physical characteristics.
2. DESCRIPTION OF THE PRIOR ART
It is well known that polyesters derived from epsiloncaprolactone are useful for the manufacture of high performance polyurethane elastomers. Polyesters of this type are described for example, in U.S. Pat. No. 3,169,945, with corresponding elastomers being described in U.S. Pat. No. 2,933,478. Unfortunately, however, the majority of the polyesters derived from epsilon-caprolactone are waxy solids melting at temperatures of from 50.degree. to about 60.degree.C. This relatively high melting point thus makes them unsuitable for many applications where a liquid polyester product is required as a starting material, as for example in the manufacture of microcellular elastomers by the "one-shot" technique, or in the manufacture of liquid cast printing ink rollers, and the like.
It is also known that the epsilon-oxycaproyl unit is capable of imparting to a polyester or a polyurethane elastomer, containing substantial portions of said unit, enhanced resistance toward hydrolysis or hydrolytic breakdown due to exposure to moist atmosphere. While the polyester products described in U.S. Pat. No. 2,933,478 are indeed much superior with regard to hydrolytic stability when compared with conventional adipate polyesters derived, for example, from adipic acid and ethylene glycol or 1,4-butanediol, they are unfortunately relatively high-melting products which limits their usefulness for the above-noted applications as well as other areas where a low melting polyester product is required.
It has been shown in the prior art that relatively low-melting liquid polyester products containing a substantial proportion of the epsilon-oxycaproyl unit may be prepared by copolymerizing epsilon-caprolactone and lower alkyl-substituted epsilon-caprolactones, or by copolymerizing epsilon-caprolactone (or a lower alkyl-substituted epsilon-caprolactone or mixtures thereof) with a dicarboxylic acid and a glycol containing one or more alkyl substituents such as propylene glycol, 1,3-butylene glycol, and the like. Such products are described, for example, in U.S. Pat. Nos. 2,933,477 and 3,139,945. However, introduction of alkyl substituents or terminal secondary hydroxyl groups into the polyester chain has been shown to result in a decrease in the strength characteristics of polyurethane elastomers derived from this type of polyester.
Consequently, there exists a definite need in the art for liquid and/or low melting hydroxyl-terminated polyester products containing a substantial portion of the epsilon-oxycaproyl unit in order to have available useful high-performance polyurethane elastomer intermediates which lend themselves to the manufacture of superior performing urethane elastomers exhibiting not only exceptional strength and toughness, but also superior resistance toward the influence of moist atmosphere at elevated temperatures.
It is further known that polyesters derived from discarboxylic acids and glycols or polyesters from epsilon-caprolactone can be utilized for the manufacture of a variety of polyurethane elastomers and for the manufacture of microcellular urethane elastomers. However, polyesters of the prior art which will result in the optimum properties of high strength and other desirable characteristics are relatively high melting waxy solids at room temperature, which limits their usefulness in many applications. A typical product which is in prominent use for the manufacture of polyurethane elastomers is the hydroxyl-terminated polyester derived from adipic acid and ethylene glycol, having a molecular weight of about 2000, and melting at a temperature of about 50-55.degree. C. Another typical product utilized in the manufacture of urethane elastomers is the homopolymer of epsiloncaprolactone initiated with a glycol, and having a molecular weight of about 2000. This product is a waxy solid melting at about 60.degree. C.
When the above polyester products of the prior art are employed in the manufacture of microcellular urethane elastomers, for example, they must first be converted to a prepolymer of an organic polyisocyanate before they can be handled satisfactorily in present processing equipment. Due to the high melting points of these polyesters, it is impractical to mix them with the organic polyisocyanate and the water required to generate the carbon dioxide in the formation of the microcellular elastomer by the so-called "one-shot" technique, since at the relatively high temperatures required to convert these polyester products to liquids, reaction rates in the "one-shot" technique become too rapid to be practical. Consequently, there exists in the art a definite need for polyester products which are essentially liquid at much lower temperatures and yet yield polyurethane products exhibiting similar strength characteristics to those observed with the above-mentioned polyethylene adipates and poly-epsilon-caprolactones.
Still a further area where the use of liquid or very low melting polyesters is of particular importance in the manufacture of urethane elastomers is in liquid cast systems, both from the point of view of the "prepolymer" technique as well as the "one-shot" technique. Liquid polyesters lead generally to liquid prepolymers upon reaction with organic diisocyanates. Liquid prepolymer products lend themselves to much more readily adaptable processing techniques than solid prepolymers. Moreover, liquid polyester products are admirably suited to the adaptation of the so-called "one-shot" technique for the formation of cast urethane elastomers. It must be noted in this regard that the polyactone-derived elastomer products have a distinct advantage over the adipic acid-derived products in that the high epsilon-oxycaproyl content of the former contributes to very substantially improved hydrolytic stability. In fact, it has been shown [Magnus et al., Rubber Chem. & Techn. 39 1328(1966] that they are as good as polyether elastomers in this respect. Consequently, there is in the field of cast urethane elastomers a definite need for novel liquid or very low melting polyesters capable of being converted to elastomers of excellent strength and having other excellent characteristics.
Still another method for the manfacture of liquid polyester products comprises reacting dicarboxylic acids with glycols which contain ether groups, for example, diethylene glycol. These urethane elastomers, however, suffer not only from decreased tensile strength, but also from decreased resistance toward humid ageing.
It is also known from the prior art that the introduction of a substantial portion of the known epsilon-oxycaproyl unit into a polyurethane elastomer provides products of excellent strength characteristics as well as products exhibiting superior humid ageing characteristics over the elastomer products derived from polyesters of conventional dicarboxylic acids and glycols. Unfortunately, however, polylactones derived from epsilon-caprolactone, hence containing the epsilon-oxycaproyl unit, show a very severe tendency to crystallize. They are even worse in this respect than polyesters obtained from dicarboxylic acids and glycols, for example, from adipic acid and alkanediols.
Consequently, there exists a distinct need in the art for liquid polyesters and the resulting polyurethanes which provide the physical characteristics outlined above.